This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third-generation partnership project
BW bandwidth
CQI channel quality indicator
DL downlink (from base station to UE)
eNB evolved Node B (e.g., LTE base station)
FSS frequency-selective scheduling
LTE long term evolution
PHR power headroom report
PL pathloss
RB resource block
RRC radio resource control
SINR signal to interference plus noise ratio
SRS sounding reference signal
TDD time division duplexing
TM transmission mode
UE user equipment (e.g., mobile device)
UL uplink (from UE to base station)
Sounding Reference Signals (SRS) are transmitted by UEs to enable an eNB to perform, e.g., frequency-selective scheduling (FSS) in the uplink or TM 7/8 beamforming in TDD downlink. The bandwidth (BW) of the SRS transmitted by a UE is determined based on the UE's transmission power capability, path loss and channel coherence time. The underlying guideline is for a UE to sound (i.e., transmit SRS) using the maximum possible bandwidth within the coherence time.
The sounding signal hops onto a different frequency band in every SRS transmission opportunity. The time period of SRS transmission is set constant for all UEs. Multiple SRS resources of the same bandwidths can be multiplexed onto the same set of subcarriers by code division multiplexing (e.g., as defined by cyclic shifts). SRS resources are identified, e.g., by a combination of the following: subframe offset, transmission comb, frequency offset of the starting point; and cyclic shift of the sequence. The subframe offset indicates, e.g., a subframe within a selected time period to use for SRS transmission. For instance, if the time period (e.g., also called periodicity) is set as 5 ms (milliseconds), there are five possible subframes (one per ms) for transmission, and the subframe offset could range from zero to four. The transmission comb (0,1) indicates whether the even or odd subcarriers are used for transmitting SRS. The frequency offset of the starting point indicates on which subcarrier the SRS resources begin. For instance, if the frequency offset is four (and assuming the transmission comb is zero), the SRS resources may start at the fourth RB group (where each RB group is a set of contiguous RBs) and include every other subcarrier for the allotted bandwidth. The cyclic shift is applied by a UE to the SRS codes used to determine SRS symbols, and the cyclic shifts provide a measure of orthogonality for SRS transmissions by multiple UEs using the same SRS resources. Typically, there are up to eight possible cyclic shifts. Additional description of SRS is provided in chapter 16.6 of Sesia et al., “LTE—The UMTS Long Term Evolution: From Theory to Practice”.
While the current SRS resource allocation methods are reasonable, they could be improved.